1. Technical Field
The present invention relates to an apparatus and method for inspecting a joint between plate edge portions having surfaces extending in opposite directions from the joint. The joint may be any one of many different types of joints, including a joint formed by aligning plates before the plates are interconnected. In one specific instance, the joint was a weld joint.
2. Background Art
A butt weld joint includes a weld between edge portions of two aligned pieces of metal. The weld has a longitudinally extending central axis and a transverse axis which is perpendicular to the longitudinal central axis. The transverse axis extends transversely to major side surfaces of the two aligned pieces of metal. The two pieces of metal are welded by either fusing the two pieces of metal together or adding a suitable weld material to form a joint with each piece of metal. The completed weld joint is usually characterized by a crown-like swelling known in the art as a weld bead.
Ideally, the major side surface of one piece of metal and the major side surface of the other piece of metal should align in the same plane after the two pieces of metal are welded. However, due to errors in aligning the two pieces of metal, one major side surface may lie in a first plane and the other major side surface may lie in a second plane. Each plane intersects the transverse axis of the weld at a different point. The strength of the weld joint depends, in part at least, on the accuracy with which the butted surfaces of the two pieces of metal are brought together. The strength of the weld joint is also determined by the geometry of the weld joint.
Weld geometry parameters, such as the peak angle and the mismatch, are often measured to determine the geometric configuration, and hence, strength of the weld joint. The peak angle is an angle formed by the intersection of the first and second planes. The mismatch is the distance between the intersections of the transverse axis with the first and second planes. A weld joint with geometries that do not meet the specific requirements of a particular specification, code, or standard is considered a defective weld joint.
Devices are available for determining the geometric configuration of a weld joint. Scales and gages employing conventional measuring methods can be used to determine the configuration of weld size and weld profile. Such scales and gages are commonly used in weld inspection operations.
A known device for determining the configuration of a butt weld joint is a contour-comb having teeth similar to a comb. The comb-like teeth are manually placed on the weld joint to obtain an impression profile of the weld joint. The impression profile is manually transferred to paper. The weld profile is then established through geometric construction using a straight edge and a protractor. The peak angle and the mismatch are manually calculated and compared with predetermined limit values. If the calculated values are within the predetermined limit values, then the geometry of the weld joint conforms to requirements and the weld joint passes inspection.
Inherent inaccuracies and poor resolution are drawbacks of the contour-comb method for determining the geometric configuration of the weld joint. The contour-comb method is unable to provide accurate and precise measurements of the geometric configuration of the weld joint. Furthermore, the labor intensiveness of the contour-comb method results in significant inspection costs.
Another known apparatus for determining the geometric configuration of a weld includes a hand-held probe assembly having three fixed probe legs and two movable probe legs. The three fixed probe legs engage a plate edge portion disposed on one side of a weld. The two movable probe legs engage a plate edge portion disposed on the opposite side of the weld. The three fixed probe legs are used to locate the probe assembly relative to the plate which they engage. The two movable probe legs are used to sense the location of the other plate relative to the probe assembly.
The probe assembly generates electrical signals corresponding to the positions of the movable probe legs. Using these signals and known geometric relationships, a microcomputer connected with the probe assembly calculates the peak angle and the mismatch of the weld joint. The microcomputer then compares the calculated values of the peak angle and the mismatch with predetermined limit values stored in a memory. If the calculated values are within the predetermined limit values, the weld passes inspection.
One drawback of this known apparatus is its lack of capability to determine the configuration of a weld joint which is longitudinally and transversely contoured. This known apparatus is capable of determining the configuration of a weld joint which is contoured in only one direction, that is in either the longitudinal or the transverse direction. Another drawback of this apparatus is the difficulty in accurately positioning the probe assembly in a direction transverse to a longitudinal axis of a weld. Still another drawback of this known apparatus is the relatively high cost of the probe assembly.